There presently exist instruments which are referred to as spectrometers. Such spectrometers may be utilized over a broad range of optical wave lengths to analyze different light sources. Basically, certain spectrometers have been referred to as infra-red spectrometers or IR spectrometers. These instruments may be scanning type instruments and utilize a dual beam or two channel system to provide at an output a ratio between the energy absorbed by a sample and the energy used as a reference. In this manner, one channel is designated as a sample channel while the other channel is referred to as a reference channel. A light source is broken up into two separate paths which are the sample and the reference paths. The light is broken up by typical optical techniques such as the utilization of prisms, lenses and so on. In such a system, the light from the sample channel is shined through a suitable aperture or lens assembly where it is directed to be absorbed by a sample. The sample used will absorb different wave lengths of light according to its characteristics. The output of the sample is eventually applied to monochromator after combination with the reference signal. The monochromator functions as an optical filter and passes a relatively narrow band of energy at a given wave length for a setting or position of the monochromator. In such a spectrometer, the monochromator is scanned at a given rate to allow the instrument to pass all energy at various wave lengths which are being absorbed by the sample. In such a system, the output from the monochromator is converted by an optical to electrical conversion detector assembly to convert the light signal from the monochromator into an electrical signal. The output of the device is basically a ratio which is representative of the light absorbed by the sample as compared to the reference light. The scanning of the monochromator may be synchronized with a pen recorder or other type display device. In this manner, the abscissa (x axis) is in wave length while the ordinate (y axis) is the ratio between absorbed light and the reference light, referred to as transmittance.
For example, if the sample absorbs no light, then the light or energy in the sample path is equal to the reference energy and one would develop a DC level at the recorder. On the other hand, if the sample absorbs 100 percent of the light then the output at the recorder would go towards zero. In this manner, as a sample is scanned one sees at the output of the recorder a series of peaks and valleys which are representative of the wave lengths and energy content which is being absorbed by the sample. If the system is scanned slowly, these peaks and valleys would change slowly. As one could see, the rate of change is a function of the scanning rate of the system. In order to eliminate detector noise, Johnson noise and other spurious signals, one would utilize a low pass filter with a relatively narrow band for a low scanning rate. The low pass filter required due to the fact that the output could be DC if there was no absorption by the sample. The band width of the filter is determined by the scanning rate and the slower the scanning the narrower the band width. This, of course, is to eliminate noise in order to obtain good resolution. It is, of course, known that such noise is broad band and is present at all frequencies. An inherent difficulty with low scanning rates is that it takes greater time to analyze a given sample.
One could, therefore, desire to increase the scanning rate in order to perform more rapid analysis. However, a rapid scanning rate requires greater band width filters as the energy level changes more rapidly. If the band width of the filter was widened arbitrarily, or by a fixed number of cycles, one begins to pass more noise. The additional noise would adversely affect the resolution of the instrument. Therefore, one would desire to change the band width of the filter for increasing scanning rates but the change has to be optimum so that a minimum amount of noise would propagate through at a given scanning rate.
The prior art contemplated a manual selection of filter characteristics. These were selected by the operator for a change in the scan rate. For example, if the selective filter network consisted of a three pole arrangement, one would conventionally require three capacitors and five resistors. To effectuate a time constant change, either all the capacitors or all the resistors would have to be changed. To obtain twelve different time constants, as in a typical spectrometer, one would therefore require switching between 60 resistors and 36 capacitors. Such a switching arrangement is, of course, both expensive and bulky and further requires continuous adjustment and selection by the operator of the analyzer.
It would therefore be desirable to provide an improved filter arrangement for use with a scanning spectrometer wherein the filter characteristics are continuously and automatically varied according to the scanning rate.